ImageMagick is the go-to image conversion library in many environments. It’s written in C and doesn’t have the best track record on security. Last year, a major vulnerability called ImageTragick (yes, there’s a logo) made the news. Even Facebook turned out to be vulnerable.
While secure alternatives exist, many existing projects have a hard dependency on ImageMagick and abstracting the image conversion can be quite involved. If you find yourself in a situation where you can’t avoid using ImageMagick, sandboxing can help you mitigate the damage in the event of a compromise.